1. Field of the Invention
The present invention relates generally to a communication device and method in a CDMA (Code Division Multiple Access) communication system, and in particular, to a device and method for assigning a channel using a quasi-orthogonal code.
2. Description of the Related Art
As one way to increase system capacity in a CDMA communication system, channelization is provided by use of orthogonal codes. The orthogonal codes can be Walsh codes. The orthogonal channelization is applied to a forward link in the IS-95 standard, for example.
Orthogonal channelization is provided to the forward link in an IS-95 communication system by establishing a transmission channel and notifying a channel receiving device of the established channel by a channel transmitting device. The following description is conducted on the assumption that the transmitting device is a base station (BS) transmitting device and the receiving device is a mobile station (MS) receiving device. Each of transmission and reception channels on the forward link is distinguished by its assigned orthogonal code. It is also assumed that each channel on the IS-95 forward link is convolutionally encoded and a modulator performs BPSK (Bi-Phase Shift Keying) modulation. The bandwidth used is 1.2288 MHz and the data rate is 9.6 kbps in the IS-95 communication system. Therefore, 64 channels (=1.2288M/(9.6 kxc3x972)) on an IS-95/IS-95A forward link are distinguished by 64 orthogonal codes, as shown in FIG. 1.
The number of available orthogonal codes is obtained after a modulation scheme and a minimum data rate are determined. The next-generation CDMA communication system will to improve system performance by increasing the number of channels available to users. To do so, the next-generation CDMA link is comprised of dedicated channels, including traffic channels and a dedicated control channel, and common channels including a pilot channel, a common control channel, and a paging channel. The traffic channels include a fundamental channel used for voice transmission and a fundamental channel used for transmission of packet data.
However, the above scheme limits the number of available channels, due to the limited number of orthogonal codes available. Consequently, the capacity of channels available to users is limited. Quasi-orthogonal codes, which provide minimized interference with the orthogonal codes, can be used to overcome the above problem. The length of an orthogonal code varies with the data rate in mobile communication systems which use a variable data rate. Therefore, it is preferable to use quasi-orthogonal codes due to its advantage of minimum interference with orthogonal codes.
However, a transmitting/receiving device in an IS-95 communication system is configured without considering the probability of using quasi-orthogonal codes for assigning a forward link channel. Thus, a different channel assignment scheme or transmitting/receiving device should be explored for application of the quasi-orthogonal codes.
It is, therefore, an object of the present invention to provide a device and method for providing channelization among forward link channels using quasi-orthogonal codes to thereby increase channel capacity in a CDMA communication system.
It is another object of the present invention to provide a channel assigning device and method for a BS in a CDMA communication system, in which the BS assigns a channel using a quasi-orthogonal code and notifies a receiving side of the assignment.
It is a further object of the present invention to provide a channel assigning device and method for an MS in a CDMA communication system, in which the MS generates a quasi-orthogonal code based on quasi-orthogonal code information included in a message received on a control channel and assigns a channel using the quasi-orthogonal code.
It is still another object of the present invention to provide a channel assigning device and method in a CDMA communication system, in which a BS assigns a transmission channel on a forward link using a Walsh code or a quasi-orthogonal code, and an MS analyzes a message received on a control channel and assigns a reception channel corresponding to the transmission channel on a forward link based on a channel identification (ID).
To achieve the above objects, a device and method for assigning a channel in a CDMA communication system is provided. According to an aspect of the present invention, in a channel assigning method for a base station, the base station checks the states of orthogonal codes and quasi-orthogonal code masks when a channel is assigned, selects an available quasi-orthogonal code mask number and orthogonal code index if it is determined from the check that a quasi-orthogonal code should be used, and generates the quasi-orthogonal code. Then, the base station assigns the generated quasi-orthogonal code to a forward link dedicated channel, generates a channel assignment message, and transmits the channel assignment message on a different channel to a mobile station.